https://www.bogleheads.org/wiki/Investment_policy_statement^explanation and includes a few real examples.
You can have an IPS as simple as a few lines or multi page cover-all-possibles one.
My IPS is pretty basic. I state what I have, what my needs are, how often I rebalance/at what levels I need to do so, and in case of X, I do Z for a few obvious scenarios like market drops or sudden influx of major $$.
IPS is your blueprint for what you have and why, and what your plans are. You can rewrite (and probably should) to adjust for significant life changes like marriage/kids/divorce/windfalls. In times of uncertainty, having a map to refer to helps you stick to the path you intended instead of panicking and running screaming off into the wilderness.